Fukushima reaches cold shutdown � Japan PM

"Even if unforeseeable incidents happen, the situation is such that radiation levels on the boundary of the plant can now be maintained at a low level," Noda said at the government's nuclear emergency response meeting.

The announcement marks a milestone after the March 11 tsunami triggered the worst nuclear crisis since Chernobyl.

The government's official endorsement of the claim by Tokyo Electric Power Company (TEPCO) that the reactors have reached cold shutdown status is a necessary step toward revising evacuation zones around the plant and focusing efforts from simply stabilising the facility to actually starting the arduous process of shutting it down.

But its assessment has some important caveats.

The government says Fukushima Dai-ichi has reached cold shutdown "conditions" - a cautious phrasing reflecting the fact that TEPCO cannot measure temperatures of melted fuel in the damaged reactors in the same way as with normally functioning ones.

Even so, the announcement marks the end of the second phase of the government's lengthy road map to completely decommission the plant, which is expected to take 30 years or more.

Officials can now start discussing whether to allow some evacuated residents who lived in areas with lesser damage from the plant to return home - although a 12-mile (20-kilometre) zone around the plant is expected to remain off limits for years to come. The crisis displaced some 100,000 people.

A cold shutdown normally means a nuclear reactor's coolant system is at atmospheric pressure and its reactor core is at a temperature below 100 degrees celsius, making it impossible for a chain reaction to take place.

According to TEPCO, temperature gauges inside the Fukushima reactors show the pressure vessel is at around 70C. The government also says the amount of radiation now being released around the plant is at or below 1 millisievert per year - equivalent to the annual legal exposure limit for ordinary citizens before the crisis began.